The present invention relates to the technical field of secondary batteries which are capable of repeatedly performing charge and discharge operations, and more particularly to a secondary battery which includes a protective circuit.
Conventionally, secondary batteries which include a protective circuit have been used with cellular phones or portable personal computers. With increasing charge capacities, there have been increasing demands for a further improved protective circuit.
Reference numeral 501 of FIG. 16 indicates a conventional secondary battery, which includes a storage battery device 511, a control circuit 515, a protective circuit 512, a main switch element 514, and a main fuse element 533.
The storage battery device 511 has a high-voltage output terminal connected to a first output terminal 528 via the protective circuit 512 and a ground output terminal connected to a second output terminal 529 via the main fuse element 533 and the main switch element 514.
The turning ON and OFF of the main switch element 514 is controlled by the control circuit 515. However, in the following descriptions, it is to be understood that the main switch element 514 is maintained in an ON state.
The storage battery device 511 is capable of repeatedly performing charging and discharging operations. When a battery charger 513 is connected between the first and second output terminals 528 and 529, a charge current flows through the protective circuit 512, the main fuse element 533, and the main switch element 514, allowing the storage battery device 511 to be charged.
An external circuit such as a portable personal computer may replace the battery charger 513. In this case, the storage battery device 511 discharges to allow a discharge current, opposed in direction to the charge current. The discharge current flow through the protective circuit 512, the main fuse element 533, and the main switch element 514 to be supplied to the external circuit.
The protective circuit 512 has a voltage sensor circuit 521, an auxiliary switch element 522, and a fuse circuit 524. The fuse circuit 524 has first and second auxiliary fuse elements 535a and 535b and a heater element 536 comprising a resistive heating element.
The first and second auxiliary fuse elements 535a and 535b are connected in series. This series circuit allows the high-voltage output terminal of the storage battery device 511 to be connected to the first output terminal 528.
A point where first and second auxiliary fuse elements 535a and 535b are connected to each other is connected to the auxiliary switch element 522 via the heater element 536.
The voltage sensor circuit 521 provides control to turn ON or OFF the auxiliary switch element 522. The voltage sensor circuit 521 detects the voltage between the high-voltage output terminal and the ground output terminal of the storage battery device 511. If the resulting voltage is less than or equal to a predetermined upper limit voltage value, the voltage sensor circuit 521 maintains the auxiliary switch element 522 in an OFF state, thereby preventing any current from flowing through the heater element 536.
On the other hand, a faulty connection of the secondary battery 501 would result in the storage battery device 511 being overcharged, thereby causing a voltage equal to or greater than the upper limit voltage value to apply on the high-voltage output terminal of the storage battery device 511. In this case, the voltage sensor circuit 521 will detect the overvoltage to turn ON the auxiliary switch element 522. As a result, a large current flows through the heater element 536 to generate heat.
The heater element 536 and the first and second auxiliary fuse elements 535a and 535b are placed in close proximity to each other. When a large current flows through the heater element 536 and generates heat, the first and second auxiliary fuse elements 535a and 535b will be blown, thereby causing the high-voltage output terminal of the storage battery device 511 to be disconnected from the first output terminal 528.
As a result, the storage battery device 511 is no longer charged, thus preventing a hazard such as generating smoke.
On the other hand, suppose that the first and second output terminals 528 and 529 are shorted out therebetween even under the normal charge condition of the storage battery device 511. In this case, the discharge of the storage battery device 511 may cause a large overcurrent, which flows through the first and second auxiliary fuse elements 535a and 535b, the main switch element 514, and the main fuse element 533.
The main fuse element 533 is placed in close contact with the main switch element 514. When the main switch element 514 breaks down or a large overcurrent flows therethrough to thereby generate heat, the main fuse element 533 will be blown due to the heat. As a result, the ground output terminal of the storage battery device 511 is disconnected from the second output terminal 529 and the storage battery device 511 is no longer discharged, thereby preventing a hazard such as generating smoke.
However, the secondary battery 501 as described above has the main fuse element 533 is located at some midpoint on the path through which a charge or discharge current flows. This raises a problem that the main fuse element 533 wastes power, causing the working hours of the secondary battery 501 to be reduced.
Furthermore, for the secondary battery 501 having a large rated current, it is necessary to employ the main fuse element 533 having a current capacity as required. Accordingly, this raises a problem that the main fuse element 533 increases in outer size and is manufactured at higher costs.
In recent years, for the portable personal computer, there have been increasing demands for reduction in size and longer operating hours, which in turn requires improvements of the secondary battery 501.
The present invention was developed to address the aforementioned conventional drawbacks.